TM 11-5805-715-34/EE119-DB-MMI-010/E154 CV3478/TO 31W2-2TTC39-12
2-4. Receive Path
(fig. FO-2)
comparator U9B will be generated and will trigger
The receive path consists of a dual level (high and
detector U4. Line loss compensation between the
low level) 2600 Hz SF receiver with logic and
NIU and the circuit switch is provided by R3 in
timing, receive gain adjust circuitry, and a 2600 Hz
conjunction with operational amplifier U1. When
band elimination filter with control to restrict the
the receiver is operating in the low-level sensitivity
signaling tone to a single trunk. The signal input
mode, high-pass filter U2 is switched in ahead of
from the circuit switch is applied on circuit card
gain adjust amplifier U3 to remove audible
NIU-A (fig. FO-4).
information tones such as ring back and busy which
are superimposed on the low-level signaling tone. If
2-5. SF Receiver
not prevented from reaching the limiter input, these
tones would capture the receiver, preventing
The purpose of the SF receiver is to detect and
recognition of the SF signaling tone and resulting in
report the presence of high- or low-level 2600 Hz
a false OFF-HOOK indication. Toggle switch S1 is
tone (SEIZE or RELEASE) from the circuit switch.
mounted on the NIU-A receive circuit card and
Initial onset of signal tone (2600 Hz) is recognized
allows the 1050 Hz test tone to be inserted into the
only as a long duration of high-level tone. Initial
SF receiver. Test points J2 and J3 at the output of
onset of low-level or of high-level tone of duration
amplifier U7 allow the signal level out of the
less than that specified is ignored by the receiver
receive card to be monitored. Toggle switch S2
logic. Voice simulation of the 2600 Hz signaling
allows the receiver logic to be manually cleared
tone is prevented by employing the limiter capture
(low-level tone being received) in the event a long
effect in the dual level (high- and low-level) SF
duration fade occurs. Because the receiver logic
receiver channel. The limiter capture technique of
only responds to a high level tone once it has
signal detection provides a precisely fixed value
recognized a valid OFF-HOOK condition, the
amplitude square wave at the output of amplifier
termination of the fade would not normally be
U11. If the square wave which is impressed upon
reported. Power on clear circuitry consisting of Q2
the 2600 Hz bandpass filter FLIB has a large
and U13 provides the same function when power is
enough 2600 Hz component, a 2600 Hz sine wave
first applied to the NIU.
of sufficient amplitude to pass the threshold level of
2-4
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